app-system: Put back support for the installed-changed signal

Use the new GAppInfoMonitor that Ryan added to glib to know when the
set of apps has changed.

https://bugzilla.gnome.org/show_bug.cgi?id=711631
app-system: Add back StartupWMClass matching

While unfortunate that we still have to scan all apps with get_all(),
support for this feature will be short-lived, so hopefully we can drop
it in the future as new apps adapt to the desktop file / app ID
recommendations.

For now, simply scan all desktop IDs.

https://bugzilla.gnome.org/show_bug.cgi?id=711631